Contemporary wireless communication systems may be characterized by multiple radio access technologies, such as Universal Telecommunications System (UMTS), Long Term Evolution (LTE) and IEEE 802.11 services such as Wi-Fi.
A multi-mode user equipment (UE) may have capabilities that are compatible with two or more radio access technologies.
In a wireless network, from a mobility perspective, a UE can be in one of three modes: connected/active, detached/inactive, or idle/sleep. By definition, when the UE is switched off or when the UE is just switched on and in the process of searching and registering with the network, the UE is in the detached mode.
In the detached mode, the location of the UE is unknown. In the active mode, the UE is registered with the network and has a radio connection with the base station, which allows the wireless network to know which cell the UE belongs to and allows the base station to exchange data with the UE.
In the idle mode, the UE does not transmit or receive data. In the idle mode, only higher level nodes in the network (for example, the Mobility Management Entity (MME) in LTE and the Serving General Packet Radio Service Support Node (SGSN) in UMTS) know an approximate location of the UE.
Routing areas (RA), in UMTS, and tracking areas (TAs), in LTE, are used to determine the approximate location of the UE. Each routing area or tracking area consists of multiple cells. In the active mode, the UE tracks its own location by recording a current routing area identifier in UMTS or tracking area identifier in LTE.
When the UE's routing area changes, the UE updates a SGSN in UMTS.
In order to reduce the number of update signaling messages from the UE to the network in LTE, a number of techniques are employed. The MME stores a list of tracking areas for each UE. If the UE simply changes tracking areas within the list, no tracking area update process is performed and no tracking area update messages are generated at the UE. This has the same effect as enlarging the number of cells included in each tracking area.
Because UMTS and other radio access technologies will coexist with LTE, and UEs are capable of handling multiple radio access technologies, LTE eliminates some of the signaling messages by assigning both a routing area and a tracking area to each UE. As a result, if the UE is moving within cells that belong to the routing area or the tracking area (with different radio access technologies, for example, UTMS or LTE), no update messages are required. When new data traffic arrives for the UE, the UE is paged by the base station in both radio access technologies simultaneously and depending on the radio access network in which the UE responds, data traffic is forwarded through that radio access network. This process is referred to as “IDLE mode signaling reduction function” in LTE and specified in 3GPP TS 23.401-v9.4.0, General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access (Release 9), March 2010.
When a call or a message for the UE arrives and the wireless network knows the UE is in the idle/sleep mode, the network uses the paging process to wake up the UE to establish a connection with the network. When the UE is in detached/inactive mode, the wireless network does not have any accurate information regarding where the UE is and, therefore, no paging process is used.